Structure for preventing loosening of threaded fasteners

ABSTRACT

A first washer is disposed on an upper surface of a member to be tightened and its projecting portion is inserted into a clearance formed between a trunk of a bolt and a piercing hole of the member to be tightened. A second washer is disposed on an upper surface of the first washer and its claw portion is met with engaging teeth of the first washer. A head portion of the bolt is fitted into a fitting portion of the second washer. The first washer and the second washer are allowed to relatively rotate in a direction of tightening the bolt which is achieved by the claw portion sliding along sloped wall surfaces of the engaging teeth and are prevented from relative rotation in a direction of loosening the bolt which is achieved by the claw portion engaging with substantially vertical wall surfaces of the engaging teeth. The bolt may not be rotated though one should try to rotate the bolt in the loosening direction after tightening since the second washer is prevented from relatively rotating with respect to the first washer in this direction.

BACKGROUND OF THE INVENTION

The present invention relates to a structure for preventing loosening ofbolts or nuts after tightening in a screw-fastening tool for tighteninga plurality of members together by using bolts or a set of bolts andnuts.

Bolts prevented from loosening through application of adhesive on screwportions of bolts or nuts prevented from loosening such as Hard Locks(trade name) or U-nuts (trade name) are conventionally known to becommercially available. However, such products are arranged in thatbolts or nuts are particularly treated or processed and are notadaptable for bolts or nuts of standard type.

Structures have further been proposed for preventing loosening of nutsin which slip surfaces are formed on bearing surfaces of nuts orsurfaces of washers such that oscillations of members to be tightened ormovements of the members to be tightened caused through thermalexpansion/shrinkage are prevented from being transmitted to the nuts.However, since large pressure is applied on bearing surfaces of nuts,costs involved for forming slip surfaces capable of standing suchpressure would lead to higher manufacturing costs.

In another known structure, square portions are integrally formed ontrunks of bolts while piercing holes of members to be tightened areformed to be of noncircular shape that fit the square portions of thetrunks. Since the fit between the square portions of trunks and thenoncircular piercing holes will restrain rotation of bolts, it will beunnecessary to fix head portions of the bolts by using wrenches orsimilar in case the nuts are to be tightened. Thus, tighteningoperations can be easily performed when employing this structure. On theother hand, such a structure is not applicable to screw-fastening toolsin which bolts need to be rotated for the tightening as inscrew-fastening tools in which, for instance, a bolt is pierced througha piercing hole of one member to be tightened whereupon this bolt isscrewed into a screw hole of another member to be tightened to achievetightening.

A structure has been suggested in which a protrusion is integrallyformed on a trunk of a bolt whereupon this protrusion is inserted into aclearance formed between a circular piercing hole of a member to betightened and the trunk of the bolt (See Japanese Examined PatentPublication No. 2-114742(1990) and U.S. Pat. No. 5,158,409). Theprotrusion of the bolt has a height that is larger than ½ of adifference between a diameter of the trunk and a diameter of thepiercing hole so that the bolt eccentrically rotates when rotatingwithin the piercing hole with the center being somewhat shifted from acentral axis of the trunk, thus making a head portion of the bolt slideon a bearing surface while continuously being shifted in a lateraldirection. On the other hand, tightening the nut upon screwing the nutinto the bolt will result in a larger sliding resistance between thehead portion and the bearing surface of the bolt so that the bolt is notallowed to further move upon shifting in lateral directions whereuponthe rotation of the bolt will be terminated. Thus, using a bolt formedwith such a protrusion will eliminate the necessity of fixing the headportion of the bolt by means of a wrench or the like when tightening thenut to thus enable easy tightening operations. However, in case it isnecessary to rotate the bolt around a central axis of the trunk for thetightening as it is the case with a screw-fastening tool in which thebolt is pierced through a circular piercing hole of one member to betightened whereupon this bolt is screwed into a screw hole of anothermember to be tightened to achieve tightening, since the one member to betightened will be shifted and moved through the protrusion of the bolteach time the bolt is rotated so that it is impossible to performtightening operations.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a structure forpreventing loosening that is also applicable to bolts or nuts ofstandard type.

It is another object of the present invention to provide a structure forpreventing loosening that can be obtained at relatively low costs.

It is still another object of the present invention to provide astructure for preventing loosening that is applicable to screw-fasteningtools of various structures.

It is still another object of the present invention to provide astructure for preventing loosening with which it is possible to achieveeffects of preventing loosening without requiring any special processesor operations after tightening.

For achieving the above object, the present invention provides astructure for preventing loosening of a screw-fastening tool comprisinga first member including a piercing hole, a second member including ascrew hole, and a bolt including a head portion and a trunk, wherein thefirst member and the second member are tightened together by insertingthe trunk of the bolt through the piercing hole of the first member,screwing the same into the screw hole of the second member andtightening the bolt, wherein a washer is disposed between the headportion of the bolt and the first member, the washer including aprojecting portion that is inserted into a clearance formed between thetrunk of the bolt and the piercing hole of the first member, wherein thebolt and the washer are allowed to relatively rotate during tighteningof the bolt while the bolt and the washer are prevented from relativelyrotating in a direction of loosening the bolt after tightening of thebolt.

The present invention further provides a structure for preventingloosening of a screw-fastening tool comprising a first member includinga piercing hole, a second member including a screw hole, and a boltincluding a head portion and a trunk, wherein the first member and thesecond member are tightened together by inserting the trunk of the boltthrough the piercing hole of the first member, screwing the same intothe screw hole of the second member and tightening the bolt, wherein afirst and a second washer are disposed between the head portion of thebolt and the first member, the first washer including a projectingportion that is inserted into a clearance formed between the trunk ofthe bolt and the piercing hole of the first member, and the secondwasher including a fitting portion that fits with the head portion ofthe bolt, wherein the first and the second washer are allowed torelatively rotate during tightening of the bolt while the first and thesecond washer are prevented from relatively rotating in a direction ofloosening the bolt after tightening of the bolt.

The above structure for preventing loosening is also applicable to a nutin a screw-fastening tool comprising a plurality of members includingpiercing holes, a bolt including a head portion and a trunk, and a nut,wherein the trunk of the bolt is passed through the piercing holes ofthe plurality of members prior to screwing the same to the nut fortightening the nut in order to tighten the plurality of memberstogether.

Since the present invention is arranged to prevent loosening by means ofwashers disposed on bearing surfaces of bolts or nuts, the presentinvention is also applicable to bolts or nuts of standard type such thatmanufacturing costs may be kept relatively low. It is also applicable toscrew-fastening tools employing bolts or to screw-fastening toolsemploying sets of bolts and nuts. It is possible to exhibit looseningpreventing effects without the necessity of performing any particularprocessing or operation after tightening. Economical advantages may beachieved since the washers may be used instead of ordinary washers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate a first embodiment of the present invention,wherein FIG. 1 illustrates a condition after completion of tighteningand FIG. 2 a condition at the time of tightening. FIGS. 1a and 2 a arelongitudinal sectional views, FIGS. 1b and 2 b plan views seen fromabove, and FIGS. 1c and 2 c vertical sectional views cut at a positionof a projecting portion of a washer.

FIG. 3 is a perspective view of the washer according to the firstembodiment.

FIG. 4 is a longitudinal sectional view illustrating a second embodimentof the present invention.

FIG. 5 illustrates a third embodiment of the present invention, whereinFIG. 5a is a longitudinal sectional view, FIG. 5b a plan view seen fromabove, and FIG. 5c a perspective view of a washer.

FIG. 6 is a longitudinal sectional view illustrating a fourth embodimentof the present invention.

FIG. 7 is a longitudinal sectional view illustrating a fifth embodimentof the present invention.

FIG. 8 illustrates a washer according to the fifth embodiment, whereinFIG. 8a is a perspective view and FIG. 8b a partially enlargedperspective view.

FIG. 9 is a longitudinal sectional view illustrating a sixth embodimentof the present invention.

FIG. 10 is a longitudinal sectional view illustrating a seventhembodiment of the present invention.

FIG. 11 is a plan view of a washer according to an eighth embodiment.

FIG. 12 is a longitudinal sectional view illustrating the eighthembodiment of the present invention.

FIG. 13 is a vertical sectional view illustrating a shape of aprojecting portion of a washer.

FIGS. 14 and 15 illustrate a ninth embodiment of the present invention,wherein FIG. 14 illustrates a condition after completion of tighteningand FIG. 15 a condition at the time of tightening. FIGS. 14a and 15 aare longitudinal sectional views, FIGS. 14b and 15 b plan views seenfrom above, and FIG. 15c vertical sectional view cut at a position of aprojecting portion of a washer.

PREFERRED EMBODIMENT OF THE INVENTION

Embodiments of the present invention will now be explained.

FIGS. 1 and 2 illustrate a first embodiment of the present invention.FIG. 1 illustrates a condition after completion of tightening and FIG. 2a condition at the time of tightening.

In FIG. 2, a trunk 1 b of a bolt 1 pierces through a circular piercinghole 2 a of a member 2 to be tightened and is screwed into a screw hole3 a of a member 3 to be tightened. A head portion 1 a of the bolt 1 isof hexagonal shape and a washer 5 is interposed between a bearingsurface thereof and an upper surface of the member 2 to be tightened.The washer 5 is formed upon stamping, for instance, a metallic plate asillustrated in FIG. 3 and comprises a flat plate portion 5 e, a pair ofarc-like rising portions 5 a rising upward from an edge of the flatplate portion 5 e, a pair of linear engaging portions 5 b provided onthe edge of the flat plate portion 5 e, a hole 5 d formed in the centerof the flat plate portion 5 e, and a projecting portion 5 c projectingdownward from an inner edge of the hole 5 d.

The flat plate portion 5 e of the washer 5 is interposed between thehead portion 1 a of the bolt 1 and the upper surface of the member 2 tobe tightened. The projecting portion 5 c of the washer is inserted intoa clearance formed between the trunk 1 b of the bolt 1 and the piercinghole 2 a of the member 2 to be tightened. As shown in FIG. 2c, a heighth of the projecting portion 5 c is larger than ½ of a difference betweena diameter of the trunk 1 b and a diameter of the piercing hole 2 a, anda center of axis 01 of the trunk 1 b is thus shifted from the center 02of the piercing hole 2 a, and the clearance between the trunk 1 b andthe piercing hole 2 a is of wedged-shape in which it becomes graduallysmaller in approaching both sides in a rotating direction. Each risingportion 5 a of the washer 5 is lower than a height of the head portion 1a of the bolt 1 and is located so as not to interfere the head portion 1a. Thus, the bolt 1 may still be relatively rotated with respect to thewasher 5 in this condition.

Tightening of the members 2 and 3 to be tightened is performed in thefollowing manner. The hole 5 d of the washer 5 is first put against thepiercing hole 2 a of the member 2 to be tightened (the projectingportion 5 c is inserted into the piercing hole 2 a) whereupon the trunk1 b of the bolt 1 is passed through the hole 5 d of the washer 5 and thepiercing hole 2 a of the member 2 to be tightened to be screwed at thescrew hole 3 a of the member 3 to be tightened. The bolt 1 is thenrotated into a tightening direction for screwing a screw portion of thetrunk 1 b into the screw hole 3 a of the member 3 to be tightened andaccordingly tightening the members 2 and 3 to be tightened together.Since the bolt 1 and the washer 5 may be relatively rotated in thiscondition, the bolt 1 rotates within the piercing hole 2 a around thecenter of axis 01 of the trunk 1 b without being restrained by thewasher 5. Thus, tightening may be performed just like with an ordinarybolt. After tightening the members 2 and 3 to be tightened through thebolt 1, the engaging portions 5 b of the washer 5 are engaged using asuitable tool while pressurizing the head portion 1 a of the bolt 1 by asuitable tool, and as shown in FIG. 2c, the washer 5 is then rotated ina direction of loosening the bolt 1 (counter-clockwise in the drawing)for press-fitting the projecting portion 5 c into the wedge-likeclearance in this direction.

Thereafter, the rising portions 5 a of the washer are swaged as shown inFIG. 1 and the swaged portions 5 a 1 are engaged with an outer peripheryof the head portion 1 a of the bolt 1. In this manner, the bolt 1 andthe washer 5 are integrated such that they cannot be relatively rotatedand loosening of the bolt 1 is prevented by the washer 5. Moreparticularly, as shown in FIG. 1c, in order for the bolt 1 to rotate ina loosening direction (counter-clockwise direction in the drawing) withthe bolt 1 and the washer 5 being integrated and the projecting portion5 c of the washer 5 contacting an inner periphery of the piercing hole 2a, it needs to rotate around the center 02 of the piercing hole 2 awhile being eccentric (the presence of the projecting portion 5 cprevents it to rotate around the center 01 of the trunk 1 b). On theother hand, in order for the bolt 1 to rotate in an eccentric manner,the flat plate portion 5 e of the washer 5 needs to slide with respectto the bearing surface of the head portion 1 a and the upper surface ofthe member 3 to be tightened while the flat plate portion 5 e cannotperform sliding movements since tightening force is applied onto thebolt 1. The bolt 1 is thus not allowed to rotate in a looseningdirection so that loosening thereof is prevented. It should be notedthat the projecting portion 5 c of the washer 5 might be dimensioned tobe short in a circumferential direction as shown in FIG. 13a or to belong in the circumferential direction as shown in FIG. 13b. By employingthe shape as shown in FIG. 13b, a rotational angle in a looseningdirection for the bolt 1 may be reduced.

FIG. 4 illustrates a second embodiment of the present invention. Thisembodiment is arranged in that the washer 5 of the first embodiment isapplied to a nut 6. The trunk 1 b of the bolt 1 pierces through acircular piercing hole 12 a of a member 12 to be tightened and acircular piercing hole 13 a of a member 13 to be tightened to be screwedinto the hexagonal nut 6. The flat plate portion 5 e of the washer 5 isinterposed between a bearing surface of the nut 6 and a lower surface ofthe member 13 to be tightened.

The nut 6 is rotated in a tightening direction for screwing the sameinto the screw portion of the trunk 1 b of the bolt 1 for tightening themembers 12 and 13 to be tightened together. Thereafter, a suitable toolis engaged with the engaging portions 5 b of the washer 5 while pressingthe nut 6 by a suitable tool, and the washer 5 is rotated in a directionfor loosening the nut 6 such that the projecting portion 5 c ispress-fitted into a wedge-like clearance between the trunk 1 b and thepiercing hole 13 a. The rising portions 5 a of the washer 5 are swagedand the swaged portions 5 a 1 are engaged with an outer periphery of thenut 6 for integrating the nut 6 and the washer 5. Since the nut 6 andthe washer 5 may be relatively rotated at the time of tightening,tightening may be performed just like with an ordinary nut, and afterintegrating the nut 6 and the bolt 5 such that they may not berelatively rotated, loosening of the nut 6 is prevented by the washer 5.The remaining matters apply correspondingly to the first embodiment.

FIG. 5 illustrates a third embodiment. This embodiment is arranged inthat the bolt 1 and a washer 15 are prevented from relative rotationthrough a screw 1 a 2 after tightening of the bolt 1. The screw 1 a 2 isscrewed into a screw hole 1 a 1 provided on the head portion 1 a of thebolt 1. Relative rotation of both members is prevented through a tip endof the screw 1 a 2 press-contacting the washer 15. As shown in FIG. 5c,the washer 15 of the present embodiment may be formed by, for instance,stamping a metallic plate, and comprises a flat plate portion 15 e, apair of linear engaging portions 15 b provided on an edge of the flatplate portion 15 e, a hole 15 d formed in the center of the flat plateportion 15 e, and a projecting portion 15 c projecting downward from aninner edge of the hole 15 d. The remaining matters apply correspondinglyto the first embodiment.

FIG. 6 illustrates a fourth embodiment. This embodiment is arranged inthat the washer 15 of the third embodiment is applied to the nut 6. Ascrew 6 b is screwed into a screw hole 6 a provided in the nut 6. Theremaining matters apply correspondingly to the first to thirdembodiments.

FIG. 7 illustrates a fifth embodiment. This embodiment is arranged inthat the washer is comprised of a first washer 7 and a second washer 8.As illustrated in FIG. 8, the washer 7 is formed by, for instance,stamping a metallic plate and comprises a flat plate portion 7 e,multiple engaging teeth 7 a provided on an edge portion of an uppersurface of the flat plate portion 7 e, a hole 7 d formed in the centerof the flat plate portion 7 e, and a projecting portion 7 c projectingdownward from an inner edge of the hole 7 d. The multiple engaging teeth7 a are aligned in an annular manner, and as shown in FIG. 8b inenlarged form, each tooth comprises a substantially vertical wallsurface and a gently sloped wall surface. The trunk 1 b of the bolt 1 ispierced through the hole 7 d. The washer 8 is formed by, for instance,stamping a metallic plate and includes a flat plate portion 8 e, a clawportion 8 b formed through notching an edge portion of the flat plateportion 8 e, a hexagonal concave fitting portion 8 a provided in thecenter of an upper surface of the flat plate portion 7 e and a hole 8 cformed in the center of the fitting portion 8 a. The flat plate portion7 e is of hexagonal shape and its linear edge forms an engaging portionto engage with a suitable tool. The claw portion 8 b is elasticallydisplaceable in vertical directions with respect to the flat plateportion 8 e. The claw portion 8 b is shaped to be somewhat warped sothat its tip end slightly projects from a lower surface of the flatplate portion 8 e in a normal condition. The head portion 1 a of thebolt 1 is fitted into the fitting portion 8 a and the trunk 1 b of thebolt 1 is pierced into the hole 8 c.

As shown in FIG. 7, the washer 7 is disposed on the upper surface of themember 2 to be tightened, and its projecting portion 7 c is insertedinto a clearance formed between the trunk 1 b of the bolt 1 and thepiercing hole 2 a of the member 2 to be tightened. The washer 8 isdisposed on the upper surface of the washer 7 and a tip end portion ofits claw portion 8 b is made to meet the engaging teeth 7 a. The head 1a of the bolt 1 is fitted into the fitting portion 8 a of the washer 8.The washers 7 and 8 may be relatively rotated in a direction oftightening the bolt 1 which is achieved by the claw portion 8 b slidingon the sloped wall surfaces of the engaging teeth 7 a but may not berelatively rotated in a direction of loosening the bolt 1 which isachieved by the claw portion 8 b engaging with the substantiallyvertical wall surfaces of the engaging teeth 7 a. Thus, tightening maybe performed just like with an ordinary bolt by rotating the bolt 1together with the washer 8 in the tightening direction at the time oftightening. On the other hand, the bolt 1 is not allowed to rotate sincethe washer 8 may not be relatively rotated in the same direction aftertightening even though one should try to rotate the bolt 1 in theloosening direction. It is accordingly possible to prevent loosening ofthe bolt 1. It should be noted that it is possible to loosen and removethe bolt 1 without causing any damages to the washers 7 and 8 byelastically displacing the claw portion 8 b of the washer 8 in an upwarddirection through a suitable tool for releasing the engagement with theengaging teeth 7 a. The removed washers 7 and 8 may be reused as theyare. The structure for preventing loosening according to the presentembodiment is also applicable to the nut side of a screw-fastening toolas shown in FIG. 4 or 6 (a screw-fastening tool arranged to tightenmembers to be tightened together through a set of a bolt and a nut). Theremaining matters apply correspondingly to the first embodiment.

FIG. 9 illustrates a sixth embodiment of the present invention. Thisembodiment is arranged in that the washer is comprised of a first washer15 and a second washer 10 with an adhesive capsule 11 being interposedbetween both members. The washer 15 is identical to that as illustratedin FIG. 5c. The washer 10 is arranged in that a hexagonal concavefitting portion 10 a is provided in the center of an upper surface ofthe flat plate portion. The head 1 a of the bolt 1 is fitted into thefitting portion 10 a. The adhesive capsule 11 is an annular memberincluding a hole through which the trunk 1 b of the bolt 1 is pierced.When rotating the bolt 1 together with the washer 10 in the tighteningdirection, the adhesive capsule 11 is compressed and broken such that anadhesive sealed in its interior is filled between the washer 10 and thewasher 15. Upon curing of the adhesive after tightening of the bolt 1,the washer 10 and the washer 15 are integrated such that they may not berelatively rotated and thus to prevent loosening of the bolt 1. Itshould be noted that the structure for preventing loosening according tothe present embodiment is also applicable to the nut side of ascrew-fastening tool as illustrated in FIG. 4 or 6 (a screw-fasteningtool arranged to tighten members to be tightened together through a setof a bolt and a nut). The remaining matters apply correspondingly to thefirst, third and fifth embodiments.

FIG. 10 illustrates a seventh embodiment of the present invention. Thisembodiment is arranged in that the washer is comprised of a first washer15 and a second washer 10′ with an adhesive 11′ being interposed betweenboth members. The present embodiment is further arranged in that aconcave adhesive holding portion 10′b is provided on a lower surface(opposing surface) of the washer 10′. Upon curing of the adhesive 11′after tightening of the bolt 1, the washer 10′ and the washer 15 areintegrated such that they may not be relatively rotated and thus toprevent loosening of the bolt 1. Since the adhesive 11′ is held by theconcave adhesive holding portion 10′b, adhesive force may bestrengthened. It should be noted that the structure for preventingloosening according to the present embodiment is also applicable to thenut side of a screw-fastening tool as shown in FIG. 4 or 6 (ascrew-fastening tool arranged to tighten members to be tightenedtogether through a set of a bolt and a nut). The remaining matters applycorrespondingly to the first, third, fifth and sixth embodiments.

FIG. 12 illustrates an eighth embodiment of the present invention. Thisembodiment is arranged in that the washer is comprised of a first washer21 and a second washer 20. As shown in FIG. 11, the washer 21 is formedby, for instance, stamping a metallic plate and includes a flat plateportion 21 e, a claw portion 21 a projecting in an outer radialdirection from an edge portion of the flat plate portion 21 e, a hole 21b formed in the center of the flat plate portion 21 e, and a projectingportion 21 c projecting downward from an inner edge of the hole 21 b.The flat plate portion 21 e is of hexagonal shape and its linear edgeforms an engaging portion to engage with a suitable tool. Engaging teeth21 a 1 are formed at the tip of the claw portion 21 a. The trunk 1 b ofthe bolt 1 is pierced through the hole 21 b. The washer 20 is formed by,for instance, stamping a metallic plate and comprises a flat plateportion 20 e, engaging teeth 20 b provided on an edge of the flat plateportion 20 e, a hexagonal concave fitting portion 20 a provided in thecenter of an upper surface of the flat plate portion 20 e, and a hole 20c provided in the center of the fitting portion 20 a. The head portion 1a of the bolt 1 is fitted into the fitting portion 20 a and the trunk 1b of the bolt 1 is pierced through the hole 20 c.

As shown in FIG. 12, the washer 21 is disposed on the upper surface ofthe member 2 to be tightened and its projecting portion 21 c is insertedinto a clearance formed between the trunk 1 b of the bolt 1 and thepiercing hole 2 a of the member 2 to be tightened. The washer 20 isdisposed on an upper surface of the washer 21. After tightening of thebolt 1, the claw portion 21 a of the washer 21 is folded back forengaging its engaging teeth 21 a 1 with the engaging teeth 20 b of thewasher 20. In this manner, the washer 20 and the washer 21 areintegrated so that they may not be relatively rotated for accordinglypreventing loosening of the bolt 1. It should be noted that, though notillustrated in the drawings, a screw may be passed through the clawportion 21 a and the same screw may be screwed into the flat plateportion 21 a for preventing deformation through returning of the clawportion 21 a after the folding. It should be noted that the structurefor preventing loosening according to the present embodiment is alsoapplicable to the nut side of a screw-fastening tool as shown in FIG. 4or 6 (a screw-fastening tool arranged to tighten members to be tightenedtogether through a set of a bolt and a nut). The remaining matters applycorrespondingly to the first and the fifth embodiment.

FIGS. 14 and 15 illustrate a ninth embodiment of the present invention.FIG. 14 illustrates a condition after completion of tightening and FIG.15 a condition at the time of tightening.

In FIG. 15, the trunk 1 b of the bolt 1 pierces through a noncircularpiercing hole 2′a of a member 2′ to be tightened and is screwed into thescrew hole 3 a of the member 3 to be tightened. A washer 30 isinterposed between the bearing surface of the head portion 1 a of thebolt 1 and an upper surface of the member 2′ to be tightened. The washer30 is formed upon stamping, for instance, a metallic plate and comprisesa circular flat plate portion 30 e, a rising portion 30 a rising upwardfrom an edge of the flat plate portion 30 e, a noncircular hole 30 bformed in the center of the flat plate portion 5 e, and a noncircularcylindrical projecting portion 30 c projecting downward from an inneredge of the hole 30 d. In this embodiment, all of the piercing hole 2′aof the member 2′ to be tightened, the hole 30 b and the projectingportion 30 c of the washer 30 are formed to be of square shape. Theprojecting portion 30 c of the washer 30 is inserted into a clearanceformed between the trunk 1 b of the bolt 1 and the piercing hole 2′a ofthe member 2′ to be tightened.

The flat plate portion 30 e of the washer 30 is interposed between thebearing surface of the head portion 1 a of the bolt 1 and the uppersurface of the member 2′ to be tightened. The projecting portion 30 c ofthe washer 30 is inserted into a clearance formed between the trunk 1 bof the bol 1 and the piercing hole 2′a of the member 2′ to be tightened.The rising portion 30 a of the washer 30 is lower than a height of thehead portion 1 a of the bolt 1 and is located so as not to interfere thehead portion 1 a. Thus, the bolt 1 may still be relatively rotated withrespect to the washer 30 in this condition.

Tightening of the members 2′ and 3 to be tightened is performed in thefollowing manner. The hole 30 b of the washer 30 is first put againstthe piercing hole 2′a of the member 2′ to be tightened (the projectingportion 5 c is inserted into the piercing hole 2′a) whereupon the trunk1 b of the bolt 1 is passed through the hole 30 b of the washer 30 andthe piercing hole 2′a of the member 2′ to be tightened to be screwed atthe screw hole 3 a of the member 3 to be tightened. The bolt 1 is thenrotated into the tightening direction for screwing a screw portion ofthe trunk 1 b into the screw hole 3 a of the member 3 to be tightenedand accordingly tightening the members 2′ and 3 to be tightenedtogether. Since the bolt 1 and the washer 30 may be relatively rotatedin this condition, the bolt 1 rotates without being restrained by thewasher 30. Thus, tightening may be performed just like with an ordinarybolt.

After tightening the members 2′ and 3 to be tightened through the bolt 1in this manner, the rising portion 30 a of the washer 30 is swaged asshown in FIG. 14 and the swaged portion 30 a 1 is engaged with an outerperiphery of the head portion 1 a of the bolt 1. In this manner, thebolt 1 and the washer 5 are integrated such that they cannot berelatively rotated. On the other hand, rotation of the washer 30 withrespect to the member 2′ to be tightened is prevented through the squarecylindrical projecting portion 30 c of the washer 30 engaging with thesquare piercing hole 2′a of the member 2′ to be tightened in a rotatingdirection. Loosening of the bolt 1 is accordingly prevented through thewasher 30. It should be noted that the shape of the projecting portion30 c of the washer 30 and that of the piercing hole 2′a of the member 2′to be tightened is not limited to a square shape but may be of anynoncircular shape that may be engaged at least in a direction ofloosening the bolt. Alternative means for preventing relative rotationbetween the bolt 1 and the washer 30 may be the structure as illustratedin the third embodiment employing a screw or the structures asillustrated in any one of the fifth to eighth embodiments employingfirst and second washers. The structure for preventing looseningaccording to the present embodiment is also applicable to the nut sideof a screw-fastening tool as shown in FIG. 4 or 6 (a screw-fasteningtool arranged to tighten members to be tightened together through a setof a bolt and a nut).

What is claimed is:
 1. A structure for preventing loosening of ascrew-fastening tool comprising a first member including a piercinghole, a second member including a screw hole, and a bolt including ahead portion and a trunk, wherein the first member and the second memberare tightened together by inserting the trunk of the bolt through thepiercing hole of the first member, screwing the same into the screw holeof the second member and tightening the bolt, wherein a washer isdisposed between the head portion of the bolt and the first member, thewasher including a projecting portion that is inserted into a clearanceformed between the trunk of the bolt and the piercing hole of the firstmember, a height of the projecting portion being larger than one-half ofa difference between the diameter of the trunk of the bolt and thediameter of the piercing hole of the first member and being smaller thanthe difference, wherein the bolt and the washer are not integratedduring tightening of the bolt such that they are allowed to relativelyrotate, while the bolt and the washer are integrated after tightening ofthe bolt such that they are not allowed to relatively rotate and thusthe bolt is prevented from loosening.
 2. A structure for preventingloosening of a screw-fastening tool comprising a first member includinga piercing hole, a second member including a screw hole, and a boltincluding a head portion and a trunk, wherein the first member and thesecond member are tightened together by inserting the trunk of the boltthrough the piercing hole of the first member, screwing the same intothe screw hole of the second member and tightening the bolt, wherein afirst and a second washers are disposed between the head portion of thebolt and the first member, the first washer including a projectingportion that is inserted into a clearance formed between the trunk ofthe bolt and the piercing hole of the first member, a height of theprojecting portion being larger than one-half of a difference betweenthe diameter of the trunk of the bolt and the diameter of the piercinghole of the first member and being smaller than the difference, and thesecond washer including a fitting portion that fits with the headportion of the bolt, wherein the first and the second washers are notintegrated during tightening of the bolt such that they are allowed torelatively rotate, while the first and the second washers are integratedafter tightening of the bolt such that they are not allowed torelatively rotate and thus the bolt is prevented from loosening.
 3. Astructure for preventing loosening of a screw-fastening tool as setforth in claim 2, wherein the first washer is provided with engagingteeth and the second washer is provided with a claw portion, and whereinthe first and the second washers are not integrated during tightening ofthe bolt such that they are allowed to relatively rotate by the clawportion sliding on the engaging teeth while the first and the secondwashers are integrated after tightening of the bolt such that they arenot allowed to relatively rotate by the claw portion engaging with theengaging teeth and thus the bolt is prevented from loosening.
 4. Astructure for preventing loosening of a screw-fastening tool comprisinga plurality of members including piercing holes, a bolt including a headportion and a trunk, and a nut, wherein the trunk of the bolt is passedthrough the piercing holes of the plurality of members prior to screwingthe same to the nut for tightening the nut in order to tighten theplurality of members together, wherein a washer is disposed between thenut and the members, the washer including a projecting portion that isinserted into a clearance formed between the trunk of the bolt and thepiercing hole of the first member, a height of the projecting portionbeing larger than one-half of a difference between the diameter of thetrunk of the bolt and the diameter of the piercing hole of the firstmember and being smaller than the difference, wherein the nut and thewasher are not integrated during tightening of the nut such that theyare allowed to relatively rotate, while the nut and the washer areengaged after tightening of the bolt such that they are not allowed torelatively rotate and thus the nut is prevented from loosening.
 5. Astructure for preventing loosening of a screw-fastening tool comprisinga plurality of members including piercing holes, a bolt including a headportion and a trunk, and a nut, wherein the trunk of the bolt is passedthrough the piercing holes of the plurality of members prior to screwingthe same to the nut for tightening the nut in order to tighten theplurality of members together, wherein a first and a second washers aredisposed between the nut and the piercing holes of the members, thefirst washer including a projecting portion that is inserted into aclearance formed between the trunk of the bolt and the piercing holes ofthe members, a height of the projecting portion being larger thanone-half of a difference between the diameter of the trunk of the boltand the diameter of the piercing hole of the first member and beingsmaller than the difference, and the second washer including a fittingportion that fits with the nut, wherein the first and the second washersare not integrated during tightening of the nut such that they areallowed to relatively rotate, while the first and the second washers areintegrated after tightening of the nut such that they are not allowed torelatively rotate and thus the nut is prevented from loosening.
 6. Astructure for preventing loosening of a screw-fastening tool as setforth in claim 5, wherein the first washer is provided with engagingteeth and the second washer is provided with a claw portion, and whereinthe first and the second washers are not integrated during tightening ofthe nut such that they are allowed to relatively rotate by the clawportion sliding on the engaging teeth while the first and the secondwashers are integrated after tightening of the nut such that they arenot allowed to relatively rotate by the claw portion engaging with theengaging teeth and thus the nut is prevented from loosening.